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Posters showing photos of and naming plain-clothes police officers working on campus recently were posted around the university area this week.
The posters were labelled "Narks in our Class?" and "Narkiology 101. How to spot a nark".
Mr Richardson said the university welcomed political debate, but did not condone law-breaking on the campus.
The university was not consulted on the use of plain-clothes officers on campus, "but would expect the police to use their normal range of powers in dealing with such a matter", he said.
The recent issuing of 12 trespass notices was related to a small number of students and non-students who had been campaigning for the liberalisation of cannabis law.
The university did not believe misuse of illegal drugs was a significant problem among its students, he said.
Dunedin and Clutha area commander Inspector Dave Campbell said yesterday three recent arrests and the issuing of trespass notices on campus were not part of an undercover police operation.
He said the arrests were made after a complaint was received just after noon on July 16 that people were smoking cannabis on the university's union lawn.
Plain-clothes police officers were sent to investigate and, as a result, uniformed officers were called in to arrest a suspect.
The plain-clothes officers were not working undercover and carried police identification.
Insp Campbell said plain-clothes officers were often used to carry out observations when police believed criminal activity might be taking place. Police were only interested in those who broke the law, he said.